Small interfering RNA (siRNA) is a short double-stranded RNA fragment with specific gene code. Generally the functional length is 21-23 nucleotides. siRNA specifically binds to complementary mRNA, which leads to the degradation and malfunction of the target. The phenomenon of post-transcriptional gene silencing mediated efficiently by siRNA is called RNAi (RNA interference), which is a defense mechanism naturally present in cells. When a double-stranded RNA enters into the cell, it is excised into siRNA fragments with 21-25 nucleotides. The fragments first bind with the other components in cells to form a nucleic acid-protein complex, which is called the RNA-induced silencing complex (RISC). The activated RISC targets a homologous mRNA by base pairing, resulting in the cleavage and degradation of the mRNA, after which cell-specific gene expression is inhibited. The siRNA is not only widely used in biomedical research, but also in the treatment of various diseases such as viral infection, cancer, vascular diseases, disorders of the nervous system and others.
Age-related macular degeneration (AMD) is one of the leading causes of vision irreversible damage in people over the age of 50 years. AMD is clinically divided into two types as “dry” and “wet”. The wet AMD develops rapidly and often results in blindness. The pathological changes of the disease cause severe visual impairment. The manifestations of AMD include the retinal pigment epithelial cells (RPE) dysfunction and choroidal neovascularization (CNV) in the macular area. Fluid leakage, RPE or neural epithelial detachment and bleeding from ruptured blood vessels can occur in severe cases. It has been found that many cellular factors play important roles in regulation in CNV generation, among which are vascular endothelial growth factor (VEGF), VEGF receptor (VEGFR), hypoxia inducible factor (HIF), angiopoietin (Ang) and other cytokines, mitogen-activated protein kinases (MAPK) and others.
RNAi has been tested for the efficacy of inhibiting CNV, Cashman et al. (Cashman et al. IOVS. 2006; 47: 3496-3504) intravitreally injected the VEGF short hairpin RNA (shRNA) constructed in adenoviral expression vectors into mice CNV model which was induced by VEGF165. The treatment effectively inhibits the over-expression of VEGF and successfully prevented the formation of CNV. The evidences showed that the shRNA adenoviral vector was delivered (transmembrane) into the cytoplasm and triggered the activation of RISC, accompanied the degradation of the target mRNA. Due to the concern of the clinical application for adenoviral vector, an attempt was made to use the naked (no transmission medium) VEGF siRNA to silence the target gene. The results indicated that inhibition of the VEGF expression via the naked “siRNA” did reduce the formation of CNV in a mice model (Reich et al. Mol. Vis. 2003; 9: 210-216).
There is still a need for improved RNA based inhibitors for the treatment of diseases such as AMD and CNV.